Basic Knowledge about Rapid Prototyping

Written by:XH Yan
Date:Friday, October 18, 2019 5:53 PM


Literally, rapid prototyping means do a prototype quickly. In practice, it gets a bit more complicated than its literal meaning. 


Rapid Prototyping for Plastic Parts

3D Printing for Plastic Parts

To make plastic parts rapidly, 3D printing is the first thing to think about.

For 1 to dozen parts, in most cases 3D printing is the best choice. We say “in most cases” is because 3D printing does have its limits. Precision for 3D printing in some cases is less than desirable. SLA process most commonly can reach tolerance about ±0.1mm. Some DLP process can reach +/-0.02 for small parts at a relatively high cost. Material choices for 3D printing is also limited, most common material for SLA is Photopolymer. MJF process could apply nylon but again at very high cost and much longer process time. ABS and PU like material also can be 3D-printed through a method called FDM but cost is relatively high. Nonetheless, materials could be applied to 3D printing are still limited. 

Among all choices of manufacturing, 3D printing suffer the worst strength. It has been claimed that FDM process can reach as much as 90% strength of mold injected plastic. SLA creates much less toughness. This is understandable as 3D printing only involved material consolidation without any pressure, while mold injection process applies high pressure when shape the parts. 


Urethane Casting for Plastic Parts

If specific material is required and that material could not be applied by 3D-printing, then the process called Urethane Casting (also known as Vacuum Casting, RTV molding and silicone molding). In this process, first a master piece is created by cnc machining, or 3D-printing. Liquid urethane or silicone then is poured around the master piece and consolidated to form a jelly-like material mold. The mold is then cut to open, replace the masterpiece with whatever the resin wanted under vacuum. There are 3 major advantages of urethane casting compared with 3D-printing. 

  • One is that more material can be applied here. 

PC-like, POM-like, ABS-like, etc. almost all ordinary plastic-like type resin can be applied in urethane casting process. Note that the resin used in this process is not totally the same material available in rod or plate form, in some cases the difference is rather obvious. For instance POM-like is not the same as POM, but similar. 

  • Another good difference is that many colors can be achieved in this process.  

Many color agent can be mixed into the resin before form the final parts. 

  • It costs less when a bit more volume of production is needed. 

A mold can handle as much 20 parts and hence it is probably the cheapest way to make 10~100 plastic parts. However, not all shape can be molded, for instance a hollow Hexahedron can be 3D-printed but cannot be silicone molded. 


Simple Injection Mold for Plastic Parts

If strength is a major concern or a bit more volume is needed, then a simple injection mold (also known as rapid injection mold) become preferable. 

This is basically a plastic injection mold. There are 3 kinds of methods to make it “simple” and “rapid”. These methods can be applied simultaneously or individually. 

  • Aluminum Mold

The first one is to use aluminum to make the mold while standard injection mold applies steel. It takes much less time to make an aluminum mold than a steel mold. When mold life-time is not a key consideration, aluminum is then a preferable material for the mold. 

  • Shared Mold Frame

Second method is to use a collective mold frame. In another words, share the mold frame with other parts, only change the mold core for different products. This could obviously save the time making the mold frame. In practice, this also means the core and the frame belongs to different owners, in most cases, cores owned by the part maker and frame owned by injection company or mold maker. 

  • Simple Mold Structure

Third thing is to apply mold structure that easier to make but harder to use, for instance, some hand pushing mechanism is applied in structure rather than auto hydraulic pushing mechanism. In other words, sacrifice the injection efficiency to the time for making the mold. Rapid injection mold is most suitable for high strength parts or products volume reaches hundreds or thousands of pieces. 


CNC Machining for Plastic Parts

And of course there is always a choice called CNC machining. Most of the plastic are available in rod and block form, so CNC machining can be applied to make a plastic part. But be noted, a part design with injection process in mind is generally costly for CNC processing. And many features can be made by injection are not made-able or too high cost for CNC processing, such as very small radius and complicated curve faces. The tolerance that can be reached highly depends on the CNC machine involved. There is a kind of CNC machine in China, specifically designed for rapid prototyping plastic parts. The machine got only one tool chuck, no tool magazine. Rigidity is low compared with CNC handling metals. Precision varies from 0.02~0.1mm, from brand to brand. And they could come with 3,4 and 5 axis. It is different from a standard CNC machine in the way that it trades-off the rigidity to cost, as plastic parts do not requires too much rigidity. And it trades-off tool stock to cost, as prototyping normally got very low volume, and speed of production is not a major concern. Thanks for the varieties of production method, we could offer more reasonable prices for all different production requirements. 


Rapid Prototyping for Metal Parts

CNC Machining for Metal Parts

Most often used process for metals parts is CNC machining. The trick here is to choose right machine for the right job. There are various CNC machines available, so are the prototyping costs. Precision needed, metal type, complicity of the parts, surface finish, hardness and volume all play a good part in the cost. There are CNC machines with low rigidity, no tool magazine, average in precision and hence low charge per hours operation time. These machines are perfect for “soft” metal such as copper alloy, aluminum alloy and Mg alloy parts without too much quantity. There are CNC machines with very tough structure, high speed spindle, 36 tool magazine. They are good for tough metal like carbon steel, stainless, hardened steel or titian alloy parts with nice finishing and high precision requirements. But of course higher charge per hours is needed. There are large groups of machines in the middle equipped with all different rigidity, tool stocks, spindle speed etc and are suitable for various requirements. From machine to machine, cost difference could be double or triple and result could be nice easy or trouble headache. Carefully choose the machine and tool is the key to obtain a cost-effective result. 

If there are part features un-made-able by cnc, EDM or wire cutting can be applied. For example, Internal sharp angle can be handled by both methods. 


3D printing for Metal Parts

Metals can be 3D printed nowadays by a process called SLM, but prices are very expensive, most metal printing price is roughly the same as buying same mass of silver or even more. And 3D metal print is also a time costly process. Average tolerance is about 0.1mm. Aluminum alloy, Stainless, Ti alloy and Ni based metal etc can be printed. 


Wire Arc Additive Manufacture or CMT (Cold Metal Transfer)

Basically, this process is for large metal parts. Most popular tolerance is mm level. CNC is normally needed for later treatment to gain a better precision and finishing. For big parts with complicated shape like airplane wings, this is a good choice. 


Hopefully, after reading this article, we hope you’ve got a general idea about what would be the best rapid prototyping for your project. If you need any support from us, please don’t hesitate to contact our account managers then.

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